The present invention relates to an improved process for the preparation of dicarboxylic anhydride from C.sub.4 -C.sub.10 hydrocarbons, such as the preparation of maleic anhydride from butane, preferably using a feed containing major amounts of alkanes, by the reaction of oxygen with the hydrocarbon in vapor phase over a particular novel catalyst.
The production of dicarboxylic acid anhydride by catalytic oxidation of hydrocarbons is well known. The current principal route for the production of maleic anhydride from C.sub.4 hydrocarbons has been desirable in the past, but is now even more desirable in view of the particular world shortage of benzene. It can be readily appreciated that direct oxidation of C.sub.4 hydrocarbons would be a hydrocarbon conservation, since for each mol of maleic anhydride prepared from benzene, one mol of benzene, molecular weight 78 is consumed, whereas for each mol of the C.sub.4, only 54 to 58 mol weight of hydrocarbon is consumed. The benzene process has consistently produced high conversions and selectivities. Although processes for the oxidation of aliphatic hydrocarbons are reported in the literature, there are certain defects and inadequacies in these processes, such as short catalyst life and low yields of product. Furthermore, although many of the prior art methods are generically directed to "aliphatic" hydrocarbons, they are in all practical aspects directed to unsaturated aliphatic hydrocarbons.
A more desirable process for producing maleic anhydride would be a direct oxidation of n-butane. There are several advantages. Principal among these is the greater availability of n-butane as compared to n-butenes or butadiene. Also, n-butenes may have higher economic petrochemical utilization than the n-butanes, which are now, often wastefully burned as cheap fuel.
In an early series of patents, Ralph O. Kerr developed a unique group of vanadium-phosphorus, oxidation catalysts, i.e., U.S. Pat. Nos. 3,156,705; 3,156,706; 3,255,211; 3,255,212; 3,255,213; 3,288,721; 3,351,565; 3,366,648; 3,385,796 and 3,484,384. These processes and catalysts proved highly efficient in the oxidation of n-butenes to maleic anhydride. Since the issuance of these pioneer patents, numerous patents have issued with various modifications and improvements over the basic discoveries set forth there, e.g., U.S. Pat. Nos. 3,856,824; 3,862,146; 3,864,280; 3,867,411 and 3,888,886.
Most recently, Kerr discovered that vanadium-phosphorus-oxygen complex type catalyst modified with a particular group of components is an excellent oxidation catalyst for the conversion of C.sub.4 to C.sub.10 hydrocarbons to the corresponding anhydrides, particularly, n-C.sub.4 hydrocarbons to maleic anhydride, which is disclosed and claimed in commonly assigned U.S. patent application Ser. No. 767,499, filed Feb. 10, 1977 now U.S. Pat. No. 4,105,586. In addition to n-butane, n-butene and butadiene may also be used as feeds. The catalyst contained only a minor amount of the modifying component. The essential elements of the modifying component were determined by Kerr to be Nb, Cu, Mo, Ni, Co and Cr. In addition to the essential elements, the modifying component may contain one or more elements from the group consisting of Y, Sm, Tb and Eu, i.e., a vanadium-phosphorus-oxygen complex type catalyst for the conversion of C.sub.4 to C.sub.10 hydrocarbons to the corresponding anhydrides in which the catalyst contains the essential modifying elements of Nb, Cu, Mo, Ni, Co and Cr, and one or more of the elements selected from the group consisting of Y, Sm, Tb and Eu. In addition to these modifiers, the Kerr catalyst might also contain one or more of the elements from the group consisting of Ce, Nd, Ba, Hf, U, Ru, Re, Li or Mg.